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Question

A bag contains 18 balls out of which x balls are red.

(i) If the ball is drawn at random from the bag, what is the probability that it is not red?

(ii) If two more red balls are put in the bag, the probability of drawing a red ball will be 98 times the probability of drawing a red ball in the first case. Find the value of x.

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Solution

Total number of balls = 18.
Number of red balls = x.

(i) Number of balls which are not red = 18 − x

∴ P(getting a ball which is not red) = Number of favourable outcomesNumber of all possible outcomes
= 18-x18

Thus, the probability of drawing a ball which is not red is 18-x18.

(ii) Now, total number of balls = 18 + 2 = 20.
Number of red balls now = x + 2.

P(getting a red ball now) = Number of favourable outcomesNumber of all possible outcomes
= x+220

and P(getting a red ball in first case) = Number of favourable outcomesNumber of all possible outcomes
= x18

Since, it is given that probability of drawing a red ball now will be 98 times the probability of drawing a red ball in the first case.

Thus, x+220=98×x18
144(x+2)=180x144x+288=180x36x=288x=28836=8

Thus, the value of x is 8.

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